Check if scrollbar and parent are visible before redraw.

diff --git a/controls/scroll.c b/controls/scroll.c
index fb31e74..f824cdd 100644
--- a/controls/scroll.c
+++ b/controls/scroll.c
@@ -565,6 +565,7 @@
     if (!wndPtr || !infoPtr ||
         ((nBar == SB_VERT) && !(wndPtr->dwStyle & WS_VSCROLL)) ||
         ((nBar == SB_HORZ) && !(wndPtr->dwStyle & WS_HSCROLL))) return;
+    if (!WIN_IsWindowDrawable( wndPtr, FALSE )) return;
 
     vertical = SCROLL_GetScrollBarRect( hwnd, nBar, &rect,
                                         &arrowSize, &thumbSize, &thumbPos );